1. Field of the Invention
The present invention relates to an insulated motor core. More specifically, it relates to a motor core coated with a multi-layer insulating film and to methods for producing the insulated core.
2. Description of the Related Art
In the prior art, it was customary to insulate motor cores using resin-based insulation coatings, powder-based coatings and electrostatically applied coatings. However, in applying such coatings, those skilled in the art encountered problems in forming films on angular areas of the core and in small crevices thereon. In instances requiring relatively thick coatings of the insulation film, motor cores have come to be insulated with the use of electrodeposition techniques. This technique is advantageous from the standpoint of rust prevention as well as for making it possible to form insulation films even on angular parts or in small crevices. (See, e.g., Toku Kai Sho 58-83559, Toku Kai Sho 58-83561 and Kokai Jitsuyo 61-205111).
As shown by these references, the performance of small motor cores is significantly affected by the thickness of the insulation film. That is, as the thickness is increased, the film-coated area of the winding becomes smaller, thereby making it necessary to restrict the number of windings. Even if the same number of windings may be used, the total length of the windings becomes greater, thereby adversely affecting the efficiency of the motor and making it difficult to reduce the size of the motor as a whole.
The above deficiencies are incapable of being overcome with the use of conventional coating techniques. Thus those who practice in the field of coating motor cores have moved toward the use of electrodeposition coating, which technique is capable of producing motors that have superior corrosion resistance. (Toku Kai Hei 01-278242)
However, even electrodeposited coatings on motor cores suffer from certain deficiencies. For example, Toku Kai Hei 8-265994 discloses a core having an electrodeposited film, which does not provide satisfactory insulation, produced by electrodeposition upon a core for a motor that drives a magnetic memory medium such as a hard disk. The problems with the coating are attributable to thickening of the coating along the edge of the core.
As described in the above reference, the tin (Sn) component of the aqueous coating solution used in the electrodeposition coating process is present in an amount of less than 12 ppm. As the amount is increased, the coating becomes thicker. The amount of carbon black is set at less than 0.5 weight per cent. An insulation film is electrodeposited to a prescribed thickness on the parts of the motor (i.e., the plastic magnet of the Nd—Fe—B system, the core made of silicon steel, and the motor base formed of die-cast aluminum) by means of cation electrodeposition coating employing an electrodeposition paint which was prepared by adding Ti02 and/or Si02 to the paint. The painted parts are heated to between 40 and 120 degrees centigrade, following which a second-stage hardening treatment is carried out wherein the parts are heated to between 150 and 190 degrees centigrade.
The reference discloses that in a hard-disk drive device prepared by assembling the coated parts, there is no scattering of the tin and no destruction of the memory. A disc drive device having satisfactory corrosion resistance and insulation properties is thus obtained.
Insulating films formed via electrodeposition coating are characterized in that, as opposed to those formed by spray coating, an electrodeposited film can be formed everywhere upon the surface of a device, including the corners, no matter how complicated the shape involved may be. Electrodeposited coatings are further characterized in that the insulation film may have a minimal thickness, thereby rendering the technique advantageous from the standpoint of permitting a reduction in size of the coated part. Nevertheless, even with the use of electrodeposition coating techniques, it is difficult to ensure that the thickness of the coated film at an angular portion of the coated device is the same as the thickness of the film on the planar portions thereof, thus rendering it substantially inevitable that the film at the aforementioned angular portions will be thinner than that on the planar portions. In view of the fact that, moreover, the tension at the time of winding is concentrated at the angular portions, the reduced thickness of the film at such angular portions, results in incursion of the winding whereupon the winding comes into contact with an uninsulated portion of the part, thereby leading to an insulation failure. Furthermore, the reduced thickness of the electrodeposited film at such angular portions often permits rusting of the coated part.
Accordingly, it is problematic in the case of an electrodeposited coating, to form a film having a uniform thickness, including upon the angular portions of the coated part, thereby rendering it difficult to reduce the size of the core, as well as that of motors containing such cores.
Further to the above, where the core is insulated solely with the use of an electrodeposition coating (which forms a coating film), if the thickness of the electrodeposited film is increased in an effort to ensure an adequate thickness on the angular portions of the core, the rate of occurrence of gas pin-holes rises, depending upon the conditions governing the electrodeposition, thereby rendering it difficult to obtain the anticipated degree of insulation. Thus, electrodeposition coatings alone do not necessarily provide a stable and reliable insulated coating film. This deficiency is demonstrated by the results of Pressure-Resistance tests on electrodeposition-coated films, the results of which are indicated below. The Pressure-Resistance test is well-known among those of ordinary skill in this art as a means of measuring resistance to passage of a current, also referred to herein as “pressure resistance” or “electrical pressure resistance”, through a coated object. The test is performed by connecting electrical leads to a coated and an uncoated surface, respectively, of an object, such as a motor core, and passing an alternating current (AC) measuring, e.g., 100, 200 and/or 500 volts (V) between the leads. Where the electrodeposited film thickness was 25 μm, the rate of failure under AC 100V pressure was 70 per cent. When the film thickness was 30 μm, the rate of failure under AC 200V pressure was 85 per cent. With a film thickness of 40 μm, the rate of failure under AC 200V pressure was 50 per cent. When the film thickness measured 50 μm, the rate of failure under AC 500V pressure was 40 per cent. The above results indicate that it is difficult, with the use of a electrodeposition coated film alone, to achieve 500V pressure resistance, even when the thickness of the film is increased.
The present invention is directed to a motor core wherein the above-described problems have been solved. In one example, the invention relates to a core with an insulating coating that is thin and has satisfactory rust-preventive and insulation properties, that does not lead to problems in connection with the winding of the slot of the motor core, and that is suitable for use in the preparation of a small-sized precision motor of high efficiency.
These objectives are achieved by the motor core of the invention in which, as described below, an electrodeposition coating is provided mainly for the purpose of rust prevention. Preferably, the insulating film coating includes a multi-layer laminated structure having: an electrodeposited layer, a comparatively soft insulation layer whose purpose is to form a uniform film and to allow production of a desired film thickness on angular portions of the coated substrate, and a relatively harder insulation layer adapted to prevent incursion due to tension at the time of winding.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.